Quarterly Financial Data (Unaudited) - Summary of Unaudited Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 30,816	$ 24,344	$ 22,676	$ 22,241	$ 23,038	$ 22,227	$ 19,058	$ 17,013	$ 100,077	$ 81,336	$ 73,112
Operating (loss) income	3,875	1,017	865	712	(2,441)	(6,250)	525	(633)	6,469	(8,799)	7,103
Net (loss) income	$ 3,554	$ 1,466	$ 1,262	$ 1,310	$ (1,753)	$ (2,682)	$ 1,534	$ (1,556)	$ 7,592	$ (4,457)	$ 3,926
Basic net (loss) income per share	$ 0.27	$ 0.11	$ 0.09	$ 0.10	$ (0.13)	$ (0.20)	$ 0.12	$ (0.12)	$ 0.57	$ (0.34)	$ 0.30
Diluted net (loss) income per share	$ 0.26	$ 0.11	$ 0.09	$ 0.09	$ (0.13)	$ (0.20)	$ 0.11	$ (0.12)	$ 0.55	$ (0.34)	$ 0.29